HIV uses a receptor-mediated pathway in the infection of host cells. HIV-1 requires contact with two cell-surface receptors to gain entry into cells and initiate infection; CD4 is the primary receptor. CXCR4 and CCR5, members of the chemokine receptor family of proteins, serve as secondary coreceptors for HIV-1 isolates that are tropic for T-cell lines or macrophages, respectively (Deng et al. (1996) Nature 381:661-6; Doranz et al. (1996) Cell 86:1149-59; and Berger et al. (1998) Nature 391:240; Feng et al. (1996) Science 272:872-877; Samson et al. (1996) Nature 382:722-725).
CXCR4 or CCR5, in conjunction with CD4, form a functional cellular receptor for entry of certain strains of HIV into cells. Recent reports indicated that the viral envelope glycoprotein gp120 interacts directly with chemokine receptors generally at a step following CD4 binding (Lapham et al. (1996) Science 274:602-605; Moore (1997) Science 276:51; Wu et al. (1996) Nature 384:179-183; and Hesselgesser et al. (1997) Current Biology 7:112-121).
Coreceptor use therefore plays a critical role in viral tropism, pathogenesis, and disease progression. HIV-1 strains transmitted in vivo generally use CCR5 (R5 viruses), whether by sexual, parenteral, or mother-to-child transmission (Fenyo et al. (1998) Nature 391:240; Samson et al. (1996) Nature 382:722-5; Shankarappa et al. (1999) J. Virol. 73:10489-502; and Scarlatti et al. (1997); Berger et al. (1998); Björndal et al. (1997) J. Virol. 71:7478-7487). These viruses typically infect macrophages and primary CD4+ lymphocytes, and do not form syncytia in vitro (Journal et al. (1997) J. Virol. 71:7478-87); they are said to be macrophage tropic (M-tropic).
Years after chronic infection is established, strains using CXCR4 (X4 strains) emerge in approximately 50% of infected individuals (Berger et al. (1998); Scarlatti et al. (1997); Koot et al. (1993); and Connor et al. (1997) J. Exp. Med. 185:621-8). This is believed to be due to the ability of X4 strains to infect primary T-lymphocytes and then to further replicate in T-cell lines and induce syncytia (Björndal et al. (1997)); they are said to be T-cell tropic (T-tropic). X4 strains not only infect an expanded spectrum of crucial target cells as compared to R5 viruses, but they also exhibit increased cytopathicity and mediate bystander killing of uninfected cells (Blaak et al. (2000) Proc. Natl. Acad. Sci. USA 97:1269-74; Kreisberg et al. (2001) J Virol. 75:8842-8847; Jekle et al. (2003) J. Virol. 77:5846-54).
Envelope variants selectively interact with either CXCR4 or CCR5. All of the known genetic determinates of coreceptor usage are found in the envelope gene (env), with the key determinates being found in the region of the env gene encoding the third variable (V3) domain of the gp120 glycoprotein. Previously, HIV-1 coreceptor utilization had been predicted according to the sequence of the V3 portion of the env gene (Hung C S et al. (1999); and Briggs D R et al. (2000)). For example, an accumulation of positively charged amino acid located in the V3 domain i.e., at positions 11 and 25 of the V3 domain and is a common feature of X4 viruses (Fouchier R A et al. (1992); Milich L. et al. (1997)). The V3 region of CXCR4-specific viruses also can exhibit greater sequence variation than their R5-specific counterparts, in particular respect with common laboratory HIV isolates at HTLV-IIIB/LAV and JR-CSF (Milich L. et al. (1997)).
The difference in cell tropism correlates with disease progression. Generally, after primary HIV-1 infection, viral populations are usually characterized by molecular heterogeneity. Strains isolated from individuals early in the course of their infection are usually M-tropic (Shankarappa et al. (1999); and Glushakova et al. (1999) J. Clin. Invest. 104:R7-R11). In many cases, the X4 and R5 strains coexist to some extent in the viral swarm or population. For example, viruses isolated from approximately 50% of individuals with advanced immunodeficiency include viruses that are M- and T-tropic. Typically, the emergence of X4 variants is associated with depletion of CD4 cells and acceleration of clinical disease. (See: Berger et al. (1998); Björndal et al. (1997); Shankarappa et al. (1999); Scarlatti et al. (1997); Koot et al. (1993) Ann. Intern. Med. 118:681-688; Connor et al. (1997) J. Exp. Med. 185:621-628; Blaak et al. (2000) Proc. Natl. Acad. Sci. 97:1269-1274). For example, it has been shown that cytopathicity toward the general CD4+ T cell population in lymphoid tissue is associated with the use of CXCR4 (Glushakova et al. (1999)). Additionally, in vitro results suggest that selective blockade of CXCR4 receptors may prevent the switch from the less pathogenic R5 strains to the more pathogenic X4 strains (Este et al. (1999) J. Virol. 73:5577-85).
Current antiretroviral therapies are intended to improve the overall clinical outcome of infected individuals. For example, treatment of infected individuals with combination antiretroviral therapy (cART), formerly called highly active antiretroviral therapy (HAART), has led to a dramatic decline in both HIV-1-related illness and death (Palella et al. (1998) N. Engl. J. Med. 338:853-60; Egger et al. (1997) BMJ 315:1194-9; Ledergerber et al. (1999) 353:863-8); Mocroft et al. (2003) 362:22-9). Early clinical trials demonstrated a reduction of plasma HIV-1 RNA loads to undetectable levels in the majority of treated individuals (Hammer et al. (1997) N. Engl. J. Med. 337:725-33; and Autran et al. (1997) Science 277:112-6). Subsequent studies, however, have showed more limited success. In particular although many patients experience initial immunologic and clinical responses to cART, the suppression of plasma viremia is not always sustained (Deeks et al. (2000); and Mezzaroma et al. (1999)).
cART has been demonstrated to preferentially suppress X4 strains during the first years of therapy, suggesting that shifts in HIV-1 coreceptor usage may contribute to the clinical efficacy of treatment (Philpott et al. (2001) J. Clin. Invest. 107:431-437; Equils, et al. (2000) J. Infect. Dis. 182: 751-757; and Skrabal et al. (2003) AIDS 17:809-814). For example, in comparison to pretherapy determinations, expression of CXCR4 was significantly increased, and CCR5 decreased, following three months of an anti-viral regimen; the changes in coreceptor expression occurred in association with a decrease in viral load and T cell activation, and an increase in naive and memory T cells, suggesting peripheral redistribution of T cell compartments (Giovannetti et al. (1999) Clin. Exp. Immunol. 118:87-94). In another study, cART was reported to reduce the expression of CXCR4 and CCR5 in lymphoid tissue (Andersson et al. (1998) AIDS 12:F123-9). These studies did not address coreceptor usage in patients undergoing HAART. The effects of cART on coreceptor usage by viral populations were heretofore unknown.
In patients undergoing cART, the predominant populations of virus shift back to CCR5-mediated entry after the CXCR4-specific strains emerge. cART may affect either the expression of CCR5 over CXCR4 or, alternatively, it may be influencing the kind of viral variant that predominates, such as CCR5-specific versus CXCR4-specific viruses. There is a correlation between the emergence of CXCR4-specific strains and rapid HIV disease progression.
Because cART is toxic to some patients, costly, and requires life-long adherence, the decision to start treatment in asymptomatic patients is complex, and therefore tailored to the individual (Yeni et al. (2004) JAMA 292:251-65). A small proportion of patients continue to experience disease progression despite cART, and questions remain regarding when to initiate and switch therapies (Egger et al. (1997) Ledergerber et al. (1999); Mocroft et al. (2003); Opravil et al. (2002) AIDS 16:1371-81; Sterling et al. (2003) J. Infect. Dis. 188:1659-65); Anastos et al. (2004) Ann. Intern. Med. 140:256-64; Mezzaroma et al. (1999) Clin. Infect. Dis. 29:1423-30; Deeks et al. (2000) J. Infect. Dis. 181:946-53; and Ledergerber et al. (2004) Lancet 364:51-61). Changing therapy in these patients, particularly after drug resistance or intolerance has developed, is also a challenge.
Currently, the principal measurements guiding therapeutic decisions are CD4 count and plasma HIV-1 RNA, as both are predictors of disease progression and response to cART (Anastos et al. (2004); Yeni et al. (2004); Ledergerber et al. (2004); Kitchen et al. (2001) Clin. Infect. Dis. 33:466-72; Egger et al. (2002) Lancet 360:119-29; and Chene et al. (2003) Lancet 362:679-86). However, debate continues about optimal treatment strategies, highlighting the need for more data to guide clinical management (Holmberg et al. (2004) Clin. Infect. Dis. 39:1699-1704; Miller et al. (2002) J. Infect. Dis. 186:189-197; and Phillips et al. (2003) AIDS 17:1863-1869). In particular, new markers are necessary to identify which patients are at highest risk for clinical disease and therefore most likely to benefit from immediate initiation or change of cART. These patients may be untreated, asymptomatic individuals or those with persistent viremia despite cART.
To accurately predict disease prognosis over time and in response to treatment, a diagnostic method would be useful to monitor the presence (or absence) of CXCR4-specific strains and/or CCR5-specific strains and shifts in coreceptor use over time. A diagnostic method for use in monitoring shifts in coreceptor use may thereby be beneficial for measuring the therapeutic efficacy of various HIV treatment regimes, such as cART. The effect of cART on coreceptor use by populations of virus has not heretofore been quantitatively studied.
The correlation between CXCR4-specific strains and rapid disease progression also indicates that a diagnostic method would be useful to monitor the presence of CXCR4-specific strains, shifts in coreceptor use associated with HIV disease progression, and to monitor the presence of CXCR4-specific strains and shifts in coreceptor use in patients undergoing antiretroviral therapy.
Accordingly, diagnostic methods for use in detecting CXCR4 isolates and/or monitoring shifts in coreceptor use (e.g. shifts from CXCR4-specific HIV to CCR5-specific HIV and vice versa) would be beneficial for predicting disease progression over time or in response to treatment. Moreover, cell-based and molecular-based methods to monitor, measure, evaluate, detect, etc. HIV coreceptor use which are reliable, accurate, and easy to use as well as being qualitative and/or quantitative in their approach would be a welcomed advance to the art.
In particular, diagnostic methods, e.g. cell-based and/or molecular-based methods, for measuring, monitoring, evaluating, detecting, etc. patient-derived HIV samples for coreceptor usage would be beneficial for evaluating HIV disease progression in the face of various anti HIV treatment and therapies.
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